TY - JOUR AB - Fluoro‑deoxyuridine monophosphate (FdUMP) is an active metabolite of 5‑fluorouracil (5‑FU) synthesized through two hypothesized pathways: The orotate phosphoribosyl transferase‑ribonucleotide reductase (OPRT‑RR) pathway and the thymidine phosphorylase‑thymidine kinase (TP‑TK) pathway. In the present study, the mechanism underlying 5‑FU resistance was investigated, focusing on changes in 5‑FU metabolism using MCF‑7, 5‑FU‑resistant MCF‑7/5‑FUR, MDA‑MB‑231 and 5‑FU‑resistant MDA‑MB‑231/5‑FUR breast cancer cells. The amount of FdUMP present following treatment with 5‑FU was determined by the density of the upper band of thymidylate synthase detected by western blotting, and its changes were investigated. MCF‑7/5‑FUR cells exhibited 5‑FU resistance (36.6‑fold), and showed decreased OPRT (‑69.3%) and TK (‑42.6%) levels. MDA‑MB‑231/5‑FUR cells also exhibited 5‑FU resistance (15.8‑fold), and showed decreased TP (‑79.0%) and increased TK (+184%) levels. MCF‑7/5‑FUR and MDA‑MB‑231/5‑FUR cells both showed decreased synthesis of FdUMP by 91 and 86%, respectively. In MCF‑7 and MCF‑7/5‑FUR cells, the synthesis of FdUMP was decreased when 5‑FU was combined with an RR inhibitor, indicating that FdUMP was synthesized through the OPRT‑RR pathway. The synthesis of FdUMP was decreased when 5‑FU was combined with a TP inhibitor in MDA‑MB‑231 cells and combined with an RR inhibitor in MDA‑MB‑231/5‑FUR cells, indicating that the synthesis pathway of FdUMP was changed from the TP‑TK pathway to the OPRT‑RR pathway on acquiring resistance to 5‑FU. Notably, the synthesis of FdUMP was increased and the resistance to 5‑FU was reversed in MCF‑7/5‑FUR cells (half maximal inhibitory concentration (IC50): 219.9 to 0.093 µM) and MDA‑MB‑231/5‑FUR cells (IC50: 157.3 to 31.0 µM) when 5‑FU was combined with a TP inhibitor. In conclusion, the metabolism of 5‑FU and the mechanism underlying the resistance to 5‑FU differed among cell lines, and inhibition of TP reversed resistance to 5‑FU, thus suggesting that the combination of 5‑FU and a TP inhibitor may be considered a promising cancer therapy. AD - Department of Breast Surgery, Gifu University Graduate School of Medicine, Gifu 501‑1194, Japan AU - Mori,Ryutaro AU - Ukai,Junko AU - Tokumaru,Yoshihisa AU - Niwa,Yoshimi AU - Futamura,Manabu DA - 2022/09/01 DO - 10.3892/ol.2022.13431 IS - 3 JO - Oncol Lett KW - breast neoplasms 5‑FU metabolism drug resistance thymidine phosphorylase PY - 2022 SN - 1792-1074 1792-1082 SP - 311 ST - The mechanism underlying resistance to 5‑fluorouracil and its reversal by the inhibition of thymidine phosphorylase in breast cancer cells T2 - Oncology Letters TI - The mechanism underlying resistance to 5‑fluorouracil and its reversal by the inhibition of thymidine phosphorylase in breast cancer cells UR - https://doi.org/10.3892/ol.2022.13431 VL - 24 ER -